Eight children who survived a coach crash in Switzerland which left 28 people dead are flying home to Belgium.

The coach was carrying 52 people back to Belgium after a school skiing trip when it struck the wall of a tunnel.

All six adults on board were killed along with 22 children. Many of the children who survived are still in hospital, three in a serious condition.

Families of some of the victims visited the scene of Tuesday night's disaster, walking into the tunnel to lay flowers.

The Belgian authorities are preparing to fly home the bodies of those who died, although nine of the children are still to be identified.

Police have cautioned against a theory in Belgian and Swiss newspapers that the driver of the coach lost concentration while trying to insert a DVD or CD.

Friday will be a day of national mourning in Belgium. A minute's silence will be observed at 10:00 GMT. The Swiss and European parliaments have already observed a minute's silence for the victims.

It was the most serious traffic accident in Switzerland for decades.

An absolutely tragic dayElio Di Rupo, Belgian prime minister

The Belgian foreign ministry said most of the children involved were aged about 12.

They had spent a week skiing in Val d'Anniviers in the Swiss Alps and were travelling home on one of three buses hired by a Christian group. The other two reached Belgium safely.

Those on board the bus that crashed were from the Stekske primary school in Lommel, near the Dutch border, and from St Lambertus in Heverlee, near Leuven (Louvain).

Most of the children killed were of Belgian nationality. Although it was originally thought that seven of the dead had Dutch nationality, the authorities in the Netherlands later revised the number of Dutch victims to six.

Among those injured are three Dutch, one Pole and one German.

DVD theory

About 100 relatives flew to Switzerland on Wednesday on a government plane to identify those who died and visit the scene of the crash.

Tributes were also being laid outside the children's schools in Belgium and books of condolence have been opened.

At the scene

By Imogen FoulkesBBC News, Sion, Switzerland

The coach hit a kerb and then swerved into what is a "stop lane" inside the tunnel. It appears to have then carried on and hit the wall at the end of the lane. Judging by the catastrophic destruction of the bus and the loss of life, this was at a very high speed indeed.

This tunnel is viewed as one of the safest - a motorway tunnel with two lanes each way and a divider so traffic is not facing head-to-head. It looks as though there were no other vehicles involved in this accident.

Although there have been safety questions over some of the more elderly Alpine tunnels, with just two lanes facing each other, this was not one of those. This was a newer tunnel with a lot of safety features - escape lanes, escape telephones, and it's very wide. There should be enough space for traffic not to hit the kerb.

At a news conference on Wednesday, Swiss prosecutor Olivier Elsig said the bus was nearly new.

It hit a concrete wall that forms part of an emergency access section head-on, in a tunnel near Sierre, in the southern Swiss canton of Valais.

Both the drivers were killed.

Mr Elsig said the speed of the bus was still being determined but there was no indication that it was travelling faster than the 100km/h (62 mph) speed limit.

The children on the bus were wearing seat belts and no other vehicle was involved, he added.

Mr Elsig said that possible causes being investigated included a technical failure, the driver suffering a health problem, or "human error".

According to the Swiss daily Aargauer Zeitung and Belgium's Het Laatste Nieuws, surviving children have said the driver was trying to put on a disk at the time of the accident.

Het Laatste Nieuws, quoting its unnamed sources, said teacher Frank Van Kerckhove, who was killed himself, had brought a disk to the front of the bus seconds before the crash.

Mr Van Kerckhove had blogged at the start of the school trip about the children watching a DVD of the Hollywood film Avatar on the coach to Switzerland.

However, Swiss police spokesman Renato Kalbermatten said CCTV from the tunnel did not confirm the disk theory, which he described as "pure speculation at this stage".

Belgian Transport Minister Melchior Wathelet said the company that ran the coach, Toptours, had "an excellent reputation".

He said: "The drivers had arrived the night before and had rested during the day before departure. It seems that the law on driving and rest periods has been respected."